Saturday, November 28, 2009

Phun Physiology: Endurance Exercise Performance — Speed or Power

Ever wonder why one endurance athlete—say, cyclist or marathoner—is faster than another, or what it might take to get faster, short of a heart and lung transplant or a new pair of genes (although we might not be able to skip the part about the pain and suffering of hard training)?

Now there is something for us academic types who are completely comfortable sitting on the sideline allegedly exercising our brains while telling other people how they can go faster by invoking pain and suffering on them.

Last year, a couple of physiologists—Joyner and Coyle—at the Mayo Clinic devised a model, which is a review of known factors, how they interact, and their ability to predict endurance performance (i.e., speed or power) in elite athletes. The authors are quick to point out there are still some important unknowns, including genetics, psychology of motivation, and aspects of neuromuscular interactions.

Thus, the model can be thought of more as a summary of current understanding, food for thought, and/or a generator of new ideas, rather than the last word on the subject. Such are models in science.

Regarding the different variables and their interaction as they relate to endurance performance, we’ve known for some time, for example, that there is more to the equation than just VO2•max , since two different athletes with the same VO2•max can possess different levels of endurance. No doubt, lactate threshold (LT) is also an important consideration in endurance performance. While many believe LT to be the most important indicator of endurance performance, this is highly contentious at best. The present model accounts for the relationship between these last two factors, or what Joyner and Coyle call “the oxygen consumption that can be sustained for a given period of time,” a concept the authors have dubbed “performance VO2.”

How other variables such as cardiac output, hemoglobin content, maximum heart rate, and the relative abundance of type I (slow twitch) muscle fibers, for example, affect endurance performance are also discussed.

The authors describe their tripartite model as follows:

VO2•max and lactate threshold interact to determine the ‘performance VO2’ which is the oxygen consumption that can be sustained for a given period of time. Efficiency interacts with the performance VO2 to establish the speed or power that can be generated at this oxygen consumption. This review focuses on what is currently known about how these factors interact, their utility as predictors of elite performance, and areas where there is relatively less information to guide current thinking.

Having been provided the various known factors governing endurance performance, an interested party can do some research to determine which ones can be improved upon and how this might be done through training.

The original article that appeared in the Journal of Physiology is located here with useful hotlinks for many of the reviewed references quoted therein.


Anonymous said...

Hi Dean,

What a great resource! As you know from some of our conversations, I'm very interested in all these things.

I didn't see this mentioned in any of the refs to the article from JoPhys., but one of the trainers at my gym says that even though we are born with certain percentages of fast and slow twitch muscle fiber (type I vs. type II), there is a third kind that, through training, can be recruited to be either slow or fast twitch.

Do you know anyting about that?

Thanks for the info!

PS. Watching the TdeF this summer I became interested in the details of reported power output for the elites...simple physics just doesn't work. I found this article interesting too.


dean furbish said...

Hi, Maria,

Thanks for the cycling physics reference regarding power which I’ve often wondered about and which I think RTP readers will appreciate!

What you heard is correct on all counts about muscle fiber (cell) types.

Allow me to summarize for the benefit of readers. There are two broad categories of muscle fibers—type I and type II, referred to as “slow” and “fast” twitch, respectively.

Endurance sports depend largely on type I muscle fibers.

Additionally, Type II muscle fibers comprise three subtypes: IIa, IIx, and IIb.

Interestingly, in spite of the fact that the types, subtypes, and percentages of muscle fibers are genetically determined, “adult skeletal muscle shows plasticity and can undergo conversion between different fiber types in response to exercise training or modulation of motoneuron activity.” (from the following source with hotlink references):

Specifically, type IIb fibers can be converted to both type IIa and type I fibers. This is fortuitous on both counts for endurance athletes, since type IIa fibers are preferred to type IIb given that the former are less prone to fatigue, while type I fibers are least prone to fatigue and are the most preferable and the type which are factored into the efficiency factor of the model featured in the bog entry.

For those who would like to learn some basics about muscle fiber types, the following reference is an excellent primer with the exception on the topic of whether muscle fiber types can be interconverted.